U.S. Pat. No. 7,850,936 discloses a system for removing SO2 from a flue gas with high efficiency having a reactor in which the SO2 contained in the flue gas is reacted with a reagent forming dust products, a separator where the dust products are removed from the flue gas and a humidifier where the dust products are humidified and forwarded back into the reactor. In order to obtain high SO2 removal efficiency, U.S. Pat. No. 7,850,936 proposes to increase the humidification rate and, in order to avoid bag binding, cold spot condensation and other problems, U.S. Pat. No. 7,850,936 proposes to install a pre-separator between the reactor and the separator, in order to remove a part of the dust from the flue gas and send the removed dust to the humidifier and thus into the reactor. The flue gas is forwarded to the separator where the remaining dust is removed and sent to the humidifier and then into the reactor as well. According to U.S. Pat. No. 7,850,936 fresh reagent is supplied into the humidified dust that is forwarded into the reactor. U.S. Pat. No. 7,850,936 specifically refers to removal of SO2, but it does not consider efficient removal of other components that could be contained in the flue gas, such as HCl.
EP 14187641.7 discloses a system for removal of components such as SO2 and HCl from a flue gas. EP 14187641.7 discloses a reactor for reacting components such as SO2 and HCl with a reagent forming dust products, a pre-separator downstream of the reactor for removing a part of the dust products, a separator downstream of the pre-separator for removing the remainder of the dust product from the flue gas. Fresh reagent is fed into the part of the dust products and the remainder of the dust products and the resulting dust products are humidified and supplied into the reactor. In addition, humidified dust products can also be supplied at a position between the pre-separator and the separator. According to EP 14187641.7 the most of the reagent (contained in the humidified dust products) is supplied into the reactor, while the reagent eventually supplied downstream of the pre-separator is a minor amount and is diluted in the humidified dust products. In the system of EP 14187641.7 the reactions occur to a higher degree (even if not only) in the reactor, because in the reactor the amount of fresh reagent allows a high reaction rate, while in the parts of the system different from the reactor the components in the dust that take part in the reactions are diluted by the components that do not take part in the reactions such that reactions cannot significantly develop.